"Africa" is the short answer to this question. But it needs explanation.

Historians can't predict black swan game-changers any better than economists can. An outbreak of plague, a nuclear holocaust, an asteroid on collision course, or just an unassassinated pinchbeck dictator at the helm of a giant military machine—any of those can have transformative effect and will always come as a surprise.

But at a macro level, it's easier to see futures, just hard to time them. The expansion of what my colleague, the great environmental historian John McNeill, calls "the human web" to build a planet-wide network of interdependent societies is simply inevitable, but it's taken a long time. Rome, Persia, and ancient China built a network of empires stretching from Atlantic to Pacific, but never made fruitful contact with each other and their empire-based model of "globalization" fell apart in late antique times. A religion-based model kicked in then, with Christianity and Islam taking their swings: those were surprising developments, but they only went so far.

It took until early modern times and the development of new technologies for a real "world-wide web" of societies to develop. Even then, development was Euro-centric for a very long time. Now in our time, we've seen one great game-changer. In the last two decades, the Euro-centric model of economic and social development has been swamped by the sudden rise of the great emerging market nations: China, India, Brazil, and many smaller ones. The great hope of my youth—that "foreign aid" would help the poor nations bootstrap themselves—has come true, sometimes to our thinly-veiled disappointment: disappointment because we suddenly find ourselves competed with for steel and oil and other resources, suddenly find our products competed with by other economies' output, and wonder if we really wanted that game to change after all. The slump we're in now is the inevitable second phase of that expansion of the world community, and the rise that will follow is the inevitable third—and we all hope it comes quickly.

But a great reservoir or misery and possibility awaits: Africa. Humankind's first continent and homeland has been relegated for too long to disease, poverty, and sometimes astonishingly bad government. There is real progress in many places, but astonishing failures persist. That can't last. The final question facing humankind's historical development is whether we can bring the whole human family, including Africa's billion, can all achieve together sustainable levels of health and comfort.

When will we know? That's a scary question. One future timeline has us peaking now and subsiding, as we wrestle with the challenges we have made for ourselves, into some long period of not-quite-success, while Africa and the failed states of other continents linger in waiting for—what? Decades? Centuries? There are no guarantees about the future. But as we think about the financial crises of the present, we have to remember that what is at risk is not merely the comfort and prosperity of the rich nations but the very lives and opportunity for the poorest.